Stability of the passive state of Al–Ta and Al–Nb amorphous alloys

The effect of Cl− ions (0–1 M NaCl) on the anodic behavior of two series of sputter-deposited Al–Ta (11–46 at.%) and Al–Nb (13–46 at.%) amorphous alloys (AAs) was investigated in neutral electrolytes by using electrochemical and in situ microscopic methods. The samples were characterized by Auger electron spectrometry; a thin alumina film was found to cover the surface of the substrate, whereas transition metal was not detected therein. In solutions containing Cl−, the alloys broke down at and above the pit nucleation potential Enp. The alloys exhibited a stable passivity within a much wider (ΔE>1 V) potential range than crystalline Al. In situ microscopic investigations revealed that for Al–Ta and Al–Nb AAs, the local current density within the pits was ∼106 higher than the current density from the remaining passive surface. The results are compared with those for Al–Mo and Al–W alloys and the mechanism of restrained passivity break down is discussed.